1. Field of the Invention
The present invention relates to a technique of starting an image forming operation and then starting feeding of a sheet and forming an image on the sheet.
2. Description of the Related Art
In a conventional image forming apparatus using an electrophotographic method, when it is checked that a sheet has been fed to a predetermined position, image formation on a photosensitive drum is started (sheet first method). Then, the electrostatic latent image on the photosensitive drum is developed, and the developed image is transferred onto the sheet (direct transfer method). On the other hand, in a color image forming apparatus using the electrophotographic method, images corresponding to respective colors formed on plural photosensitive drums are temporarily transferred onto an intermediate transfer member such that they are superimposed on each other. Then, a full color image where the plural color images are superimposed is transferred onto a sheet (intermediate transfer method). The intermediate transfer method, proposed in view of occurrence of color shift due to variations of materials of sheets, has become popular.
However, in the above intermediate transfer method, when sheet feeding is started and then image formation processing on the respective photosensitive drums is started, the time period from the start of image formation operation to the transfer is long, therefore the printing efficiency is not improved. Accordingly, in the intermediate transfer method, the printing efficiency is improved by adopting a first image formation method of starting image formation processing on the photosensitive drums before start of sheet feeding.
In the former first sheet method, as the image formation processing is started on the presumption that the sheet has been fed to the predetermined position, variations in feeding time in accordance with type of the sheet or the like can be easily handled. On the other hand, in the latter first image formation method, as the image formation processing is started before it is checked whether or not the sheet feeding has been normally performed, there is a probability that the sheet feeding cannot follow the image formation processing.
As a technique for detecting such inconvenience upon sheet feeding, Japanese Patent Application Laid-Open No. 03-279142 proposes execution of sheet feeding retry upon occurrence of paper separation failure. Further, Japanese Patent Application Laid-Open No. 10-123908 discloses, in reading and duplication of an original, stopping an original reading operation when sheet feeding is delayed. Further, Japanese Patent Application Laid-Open No. 05-221561 discloses, in continuous sheet feeding, suspending an image formation operation in a current image formation cycle upon occurrence of delay of sheet feeding, instead of performing processing in the case of paper jam, and then when the sheet arrives at an image forming position in the next image formation cycle, continuing the image formation operation.
However, in these conventional techniques, as image formation by the sheet first method is presumed, the techniques are not appropriate for image formation by the first image formation method. That is, in the above-described first image formation method, it is presumed that a sheet follows a formed image on a photosensitive drum or an intermediate transfer member. However, a trouble occurs when a sheet cannot follow a formed image due to variations of sheets, which gets recognized as paper jam.